The AM Forum

What's the difference between a fuse that is rated at 125V at say 5amps and a fuse that's rated at 32v at 5 amps?  Five amps is 5 amps right?

the 125 @ 5 is 625 watts. The 32 @ 5 is 160 watts.

If you look at the high voltage fuse you will see construction that allows some space to eliminate arcing when the fuse blows. These are usually constructed with fine wire, maybe an inch of it. When the fuse blows, there is a large space between the two contacts that held the gazinta and gozota connections. If you look at a low voltage automotive fuse you will see the fuse element is a heavy piece of metal that has been machined in the center down to the proper size for the rated current. When a low voltage fuse is used in a high voltage (120-240 volt) application the center will open but the rest of the element will be there to keep an arc going and not properly open the circuit. In practice I have used 32 volt fuses in 110 volt transformer primaries and had no problems but I would not try that on 240 volts.
Keith
WA1HZK

Low voltage fuses have a lower series resistance

Well, WA1GFZ,
   I doubt that low voltage fuses have a lower series resistance than higher voltage fuses. (current is current). Resistance is a necessary evil of fuses. If it wasn't for the internal resistance, the fuse would never blow.

As far as small 1/4" glass fuses like Buss AGC and Littlefuse 312 types, I think the difference in voltage ratings is merely the markings on the end caps.

In a 120 volt circuit, I would have no qualms whatsoever with using a 32 volt rated fuse and would certainly be comfortable with a 125 volt fuse in a 12 volt mobile installation.

It does become significant when you're talking high voltage. If you're fusing a 7200 volt line circuit, I would certainly use a high voltage fuse that would break and leave a wide gap.

Anyway, any fuse is better than nothing.

                                           John  (pui)

I conducted an unscientific survey of my junkbox fuses with my HP 4261A digital L-C-R bridge, and it appears that Frank is correct.

Subtracting for meter lead resistance, here are my results:

3/8A, 250V slo-blo - 2.5 ohms
1/2, 250 - 1 ohm
3/8, 250 slo blo - 4.5 ohms
1/2, 250 fast - .1 ohm
4A, 250, .02 ohm
4, 250, .04 ohm
8, 250, .02 ohm
4A, 32V - .01 ohm
9, 32 - .01 ohm
2A, 125V fast - .1 ohm

There is a marked difference in style of the fusible element between 32V and higher voltage fuses.  As noted above, the low voltage fuses have a wide strap with a notch cut in the middle.  The higher voltage fuses have a spring on one end or have a zig-zag style element - presumably to ensure that there is no arcable distance once the fuse blows.  I note that blown low-voltage fuses just leave a small gap at the notch, where blown high voltage fuses vaporize the element or, due to the spring-loaded feature, leave a gap of almost the length of the fuse.

My conclusions:  using a 2A/250V fuse with a resistance of .1 ohms in a low voltage circuit (say 1 amp continuous) is going to provide a voltage drop of .1 volts.  Not a good idea in a 12V  or 5V regulated circuit.  You pays lotsa money and take lotsa time to design a good regulator and then your fuse, of all things, drives the regulation all to hell.

And I sure wouldn't use a low voltage fuse, where burning a small gap in the notch when the fuse blows, in a high voltage (120-250V) circuit.  It just doesn't  appear that there's enough of a gap to ensure arc quenching.

And under NO circumstances would I use a 250V fuse in a higher voltage circuit, *ESPECIALLY* with a chassis-mounted fuse holder.

Guys, this is how fires start and how hams get killed, by JSing around VERY SIMPLE AND INEXPENSIVE safety features.  THe proper fuse costs all of a quarter.  Why on earth would you ever compromise this?

73 John

A quarter?!!  We hams are cheap bastards and would rather spend a PENNY -  stuck in the socket...   :lol:

Good post, John.  Nothing like some test data to learn the real story. TNX for the extra effort, OM.

T

I also did a study down on the test bench. After searching through hundreds of fuses to see if I could find 2 fuses of the same value and at the 2 different voltage markings, I found  two 15 amp 3ag fuses. One is a 32v and the other a 250v.

I used a mil spec fuse holder to conduct this measurement and repeated the test several times.
Here is the results of my test on a HP 34401A meter using the 4W measurement technique:

The 15 amp @ 32v =   .015 ohms

The 15 amp @ 250V = .016 ohms

The meter read .001 with all 4 leads shorted together.

So, my results indicate that (in the 15 amp range anyway) the two fuses were identical in resistance.

Interesting.  I had once built a power supply and put a low-current 250V fuse in the line... and the series resistance of the fuse was surprising!  I assumed 32V fuses had lower series resistance, and maybe 250V fuses were supposed to blow wider apart.  But looking at the series resistance specs from a couple of Bussman fuse types, the series resistance seeems to be a function of the current rating, not the the voltage rating.

I'm surprised!  I always associated 32V fuses with low series resistance.

TNX, T!  Many of the slo-blo fuses in the junkbox actually have little 1/4 watt resistors in them, so you can expect a bit more series resistance from a slo-blo than a fast blow.   The resistance of the high voltage/slo-blo fuses would definitely make a difference at low voltage.  Of course, for solid-state work you don't wanna use slo blos anyway.

73 John

I suspect the lower current rated fuses will have a greater resistance difference between low and high voltage units.
So Tom Vu how many amps per penny?
Lots of Police on Rt. 2 down your way tonight....